1. Field of the Invention
The present invention relates to an electroless plating process capable of selectively forming a plating film on the surface of a workpiece and a process for producing a multilayer wiring board utilizing the electroless plating process. Particularly the present invention relates to a selective electroless plating process suitable for formation and correction of a minute pattern by a plating film, and a process for producing a multilayer wiring board according to which a connector part is formed by the mask-less utilizing the above electroless plating process.
2. Prior Art of the Invention
The electroless plating techniques have been employed for the plating of various kinds of insulators, semiconductors, etc. Recently, the electroless plating techniques have begun to be employed in formation and correction of a conductor pattern on a circuit board, etc. Thus the selective electroless plating techniques, which is employed the plating on only a predetermined portion of the surface of the circuit board, have attracted attention.
One of the kind selective electroless plating techniques is a process comprising irradiation with a radiation energy beam. (U.S. Pat. No. 4,239,789).
In this conventional electroless plating process, a workpiece is immersed in an electroless plating bath filled in a container. The portion of the surface of the workpiece where a plating film is to be formed is irradiated with a laser beam through a modulator or a condenser lens from an energy source. As a result, the portion of the workpiece which is being irradiated with the laser beam is heated, and a plating film is selectively formed on only the portion thus heated.
The selective electroless plating process is applied to formation of a conductor pattern on a circuit board, etc. Since the surface of a workpiece is different in substance from ions to be deposited thereon, the surface of the workpiece must be preliminarily activated by, for example, immersing the workpiece in a solution of palladium chloride before the selective plating treatment.
Accordingly, in the conventional selective electroless plating technique, there has been a problem that sufficient selectivity by an energy beam is not attained since a plating film is formed all over the activated surface when the workpiece is merely immersed in the electroless plating bath.
One of the selective electroless plating processes is a process as disclosed in the Japanese Patent Laid-Open No. 64368/1983, in which silica for IC is used as the workpiece and the following electroless plating bath is used:
(HCHO).sub.n : 0.1 mol/cm.sup.3 PA0 CuSO.sub.4 : 0.05 mol/cm.sup.3 PA0 EDTA.disodium salt: 0.15 mol/cm.sup.3 PA0 NaOH: adjustment of pH to 12.5
Here, EDTA is ethylenediaminetetraacetic acid.
Accordingly, ions present in the plating bath are Cu which is different from silica as the workpiece. Thus the workpiece must be subjected to the following activation treatment.
Pretreatment is first carried out which consisting of water washing, a cleaning treatment with an about 10% solution of NaOH, water washing, an acid washing treatment with an about 3.5% solution of HCl, water washing, and drying. Subsequently, the workpiece is subjected to a sensitization treatment with a treating liquid composed of about 10 g/l of SnCl and about 20 to 50 g/l of HCl, followed by moderate water washing.
Subsequently, the workpiece is subjected to an activation treatment with an activating liquid of about 0.2 g/l of PdCl.sub.2 adjusted to a pH of 3 to 4 with a pH adjustor HCl at about 50.degree. C. to substitute Pd for Sn. Thus a substance, which serves as plating nuclei in chemical plating, is stuck to the workpiece. The resulting workpiece is then washed with water, dried, and, if desired, cooled and stored.
Subsequently, the workpiece thus activated is immersed in an electroless plating bath having the above-mentioned composition, and irradiated, on the surface to be plated, with a laser beam emitted from an about 15 W argon laser and condensed to a diameter of 15 .mu.m. A plating film is formed in the portion irradiated with the laser beam. Thus selective electroless plating is effected.
However, where the workpiece activated is immersed in the electroless plating bath having the above-mentioned composition, electroless plating is allowed to progress to some extent at ordinary temperatures even if the workpiece remains as it is, leading to deposition of a plating film. Therefore, in the conventional selective electroless plating technique, the plating path and the workpiece must be kept at around 5.degree. C. or a lower temperature, thus presenting a problem of a troublesome plating treatment.
Furthermore, since the surface of the workpiece is already activated all over, deposition of a plating film cannot be completely suppressed even if temperature control as mentioned above is made, thus presenting another problem of insufficient selectively.
As described above, in the selective electroless plating wherein the surface of the workpiece is different in substance from plating ions in the electroless plating bath, the surface has conventionally been subjected to the activation treatment before the plating treatment. This procedure aims at preliminarily adhering fine particles, acting as nuclei of growth of a metal to be deposited and, hence, as the catalyst of the plating reaction, on the surface of the workpiece. The plating reaction has conventionally been initiated by the nuclei.
It has been believed that the plating reaction cannot be controlled without this activation treatment. Thus this treatment has always been effected in the conventional selective electroless plating technique. A confirmative experiment conducted by the inventors of the present invention provided that, as a result of the activation treatment, deposition of a thin plating film was observed all over the surface of the workpiece where the activation treatment was effected, even in the portion not irradiated with the energy beam when the temperature of the plating bath was normal, and that, even when the bath temperature was 5.degree. C. at which electroless plating was not believed to progress substantially, deposition of a plating film in an innegligible amount was observed over a considerably wide area of the portion not irradiated with the energy beam.
Further, according to the prior art selective electroless plating technique, a selective chemical or thermal reaction is effected on a workpiece by using a laser beam to form a pattern on the workpiece. At this time, the workpiece is not affected at all. More specifically, active sites are imparted to the workpiece by an activating agent, and a plating film is formed on the active sites. Therefore, the material itself of the workpiece is not changed at all before and after irradiation with the laser beam.
A printed circuit board is used for mounting parts on an electronic apparatus and efficiently carrying out wiring between the parts. The printed circuit board shows a general trend toward a multilayer structure for improving the packaged density. There are various methods for bonding between the layers of the multilayer board.
The procedure of layer build-up is as follows. A photoresist is applied on a carrier metal, followed by pattern light exposure and development. Subsequently, Au, Cu, or the like is deposited on the metal, followed by removal of the resist. Etching is effected to form a plated pattern. Subsequently, a polyimide film is formed to flatten the surface. Thus the first layer is formed.
This process is repeated to form a multilayer circuit board. Formation of the interlayer portion between the first layer and the second layer requires repetition of the same process three times. This is because the height of a plating film built up at one run is, for example, 30 .mu.m or less as against 100 .mu.m necessary for a distance between the layers.
Thus the procedure of layer build-up is an effective method capable of coping with the trend toward a higher packaging density of the printed circuit board. However, there is a problem that the procedure involves an increased number of steps, particularly a large number of steps for providing connector parts between the layers.